Electrical stimulation of the brain is under increasing use for such varied purposes as relief of chronic pain and treatment of movement disorders. A typical electrical brain stimulation system comprises a pulse generator operatively connected to the brain by a lead. The lead has an electrode at its distal end, designed to be implanted within the patient's brain, and a connector assembly at its proximal end, designed to connect to the pulse generator. Thus an electrical signal from the pulse generator is transmitted through the lead to the electrode and thus to the desired site in the patient's brain. Access to the desired position in the brain is generally accomplished by drilling a hole in the patient's skull or cranium.
Typically, a cranial drill (commonly referred to as a burr) is employed to provide access through the skull. Occasionally drilling through the skull may cause bleeding proximate to the hole. In such a situation a common technique is to redrill the hole in a larger diameter, thereby uncovering the bleeding vessel. Once uncovered the surgeon may treat the bleeding vessel.
Once a satisfactory burr hole has been achieve through the skull the desired stimulation site is located. Next the stimulation lead is placed with at least one electrode positioned at the desired stimulation site.
Typically the stimulation site is located and the lead electrode is positioned using a stereotactic instrument, such as that disclosed in U.S. Pat. No. 4,350,159 to Gouda, incorporated herein by reference. Use of such an instrument permits very precise movement within the brain, crucial to prevent unintended injury. Once the lead is positioned and tested to determine that the results of stimulation are satisfactory, it is critical that it not be moved. Even one millimeter of electrode travel may cause unsatisfactory results or even injury to the brain. As can be appreciated, traction on the portion of lead positioned outside the skull could easily cause movement of the portion positioned within the brain. Thus, reliable anchoring of the lead within the burr hole is necessary.
Previous designs of burr hole lead anchors required disconnecting the lead from the stereotactic instrument before the anchor could be positioned. For example, a burr hole lead anchoring system disclosed in U.S. Pat. No. 4,328,813 to Ray, incorporated herein by reference, consisted of an annular socket and cap to anchor a lead within a burr hole. Specifically the cap was positioned within the annular socket so the lead is trapped by the frictional fit between the socket and cap. This system, however had several drawbacks. First the lead was secured off center form the burr hole and thus could not be supported by the stereotaxic instrument during installation. Because the lead is unsupported while the anchor was installed, the lead was much more susceptible to dislodgement. Moreover the design of the plug and socket, in fact, have been found to cause dislodgement, specifically the lip of plug as it engages the socket tends to pull or dislodge lead. This design, moreover, does not have the ability to be fitted into various sized burr holes. As mentioned above, this can be a serious drawback, as a common technique to control bleeding within the skull, caused during the drilling of the burr hole, is to redrill the hole to a larger diameter and expose the ruptured vessel.